Frequency modulation



April 8, 1941. G. R. CLARK FREQUENCY MODULATION Filed May 12, 193@ 'veur 1"" INVENTOR 6u .ez-'P7' A. amm( Patented Apr. 8, 1941` UNITED STATES PATENT orifice FREQUENCY Monum'rroN Gilbert n. clerk, nmkiyn, N. Y., signor to Badin Corporation of America, a corporatlonof Delaware Application May 12, 193s, serai No. 273,175

a claims. (ci. 25o-21) Y quency demodulator is a filter whose voltage attenuation varies approximately linearly over the frequency range used in the modulation.

The filter type of frequency demodulator places certain requirements on the apparatus associated therewith. For example, the frequency input to the filter must be quite free of amplitude variations so that the output amplitude will be a function solely of the input frequency. Also, the input frequency to the filter demodulation should be virtually a pure sine wave since the higher frequency components present in va distorted Wave may fall within the useful portion of the filter characteristic and influence the demodulated output adversely. .These requirements can be met with suitable design of apparatus but in my invention li avoid this diificulty by use of an en tirely different type of demodulator.

It is one object of my invention to produce a frequency demodulator that is independent of any amplitude modulation of the sub-carrier frequency.

Another object is to produce a frequency demodulator that requires no amplitude limiters or automatic gain control devices in any part of the receiving system.

Another object is to produce a frequency de modulator in which the output is related absolute ly linearly to the input frequency;

Another object of my invention is to produce a frequency demodulator that will resppnd only to the fundamental component of the sub-carrier frequency, regardless of distortion of the wave shape of this frequency.4

Another object is to produce a frequency clemodulator in which square wave keying can 1be used for the sub-carrier frequency.

Other objects will appear in the following description, reference being had to the drawing in whichA Fig, 1 is ablock diagram which my frequency demodulator may be used.

Fig. 2 is a block diagram of the receiver containing my demodulator as a unit thereof.

Fig. 3 is a diagrammatic illustration of the circuit of the demodulator.

Referring to the drawing, the picture scanner I or other signaling device varies the frequency of a generator 2 to produce a sub-carrier of low lfrequency. For example, a suitable band for il facsimile transmission has a range of 1600 to 2000 of a ltransmitter with l cycles per second but various other bands either above or below this may-be used Awith like results. 'Ihe method bywhich the sub-carrier frequency is varied to produce the modulation is immaterial as far as my invention is concerned and it is not V described in detail, but it may be produced, for example, by the particular arrangement disclosed inmy'joint application with James E. Smith and James N. Whitaker, Serial No. 270,332, led April 27,1939. The. output of the frequency modulator 3 will be used to amplitude-modulate the radio frequency carrier of the generator 4. This modulation may be obtained by any of the well known methods'and the details need not be described. The output of the modulator 5 is passed to the transmitter E', which may be of any desiredconstruction and the signal is radiated by any suitable antenna.

In the receiving apparatus of Fig. 2 the signal is received on the antenna I and is amplified, detected and further amplified at 8. The output of receiver 8 is passed through low pass filterV 9 which in this case may have an upper limit of slightly above 2000 cycles, but this may be omitted when the sub-carrier frequency chosen is in the order of to 200 cycles. The output lines ,i0 of low pass filter B are connected to my improved frequency dernoduiator or converter I I for changing the frequency modulations into amplitude modulations. ulator are connected to another low pass filter i3 which with the hand given as an example would have an upper limit of less than i) cycles per second so as to spread the effect ofthe constant amplitude pulses of demodulator il over the complete cycle. y

The output of the low pass filter it passes into recorder I4. The receiving apparatus 3, low pass iilters 9 and it and the recorder i4 may be any of the well known constructions and the details are not disclosed as they are not necessary for an understanding of myv invention.,

The details of the frequency demodulator I l are shown in` Fig. Ji. In this figure the input lines I0 are connected to the primary of a transformer, the secondary of which has sections 15 and I6. Section I5 of the secondary has one terminal connected through resistance I1 to the grid of a Thyratron or grid controlled gas tube I8. The other terminal of this secondary is connected through resistance I9 and resistance 20 to the cathode 2| of tube I8. one terminal connected through resistance 2l to the grid of a Thyratron tube.23 and its other terminal connected through resistance I9 and resistance 24 to cathode 25 of tube 23.

The secondary I5, I6, is arranged to feed the The output lines It of the demod- Thesecondary I6 has grid circuits of the two Thyratron tubes in phase opposition as in an ordinary push-pull circuit.

The anode 28 of Thyratron tube I8 and the anode 21 of Thyratron tube 23 are connected in parallel to the positive terminal 28. The negative terminal 23 of the supply source is grounded at and is connected to the mid-terminal connection 3l of the transformer as shown. Filter condenser 32 may be connected across the resist- The cathode 2 I of tube I8 is connected through condenser 34 and resistance 35 to ground at 36, and the cathode 25 of tube 23 is similarly connected through condenser 31 and resistance 38 to ground at 36. These two condensers and their associated resistances should form circuits having a suiiiciently short time constant to permit them to charge or discharge completely within a half cycle of the highest frequency of the varying subcarrier.

Double rectifier 33 has one anode 40 connected to the ungrounded end of resistance 35 and the other anode 4| connected to the unground'ed end of resistance 38. The cathode of this rectifier is connected through resistance 42 to ground at 43.

This cathode is also connected through resistance 44 to grid 45 of tube 46 which serves to amplify the output of the rectier 39. The cathode 41 of this tube is connected to the junction point of resistance I9 with the resistances 20 and 24, and the anode 48 is connected to one output terminal I2, the other output terminal I2v being connected to positive terminal 28 of the source of supply.

The operation will now be described.

The scanner l of Fig. 1 will produce a variable amplitude output and this varies the frequency of the sub-carrier source 2. The varying frequency output of source 2 covering a band, as assumed, 1600 cycles to 2000 cycles, passes into modulator 5 and the amplitude of the radio frequency carrier of generator 4 is modulated by this varying sub-carrier frequency. The output of the modulator 5 passes to the transmitting apparatus S and is radiated.

At the distant receiver the antenna E intercepts the modulated radio frequency carrier wave and this is amplified and detected to extract the sub-carrier frequency and is further amplified. The sub-carrier of course has the modulations of the signal in its varying frequency and it in most cases will also contain undesired amplitude modulations imposed by varying conductivity of the upper ionosphere. The varying sub-carrier frequency may also contain higher frequency components from other causes. The latter are removed by the low pass filter 3.

The output of the low pass filter is introduced into the transformer secondaries I5, I6, and on account of their connection in the way described, the grids of gas tubes I8 and 23 will be alternately positive and negative. If, for any given instant we assume that gas tube 23 is conducting, commutating condenser 33 will be charged by the voltage drop in resistance 24, the lower end of the condenser being positive, as shown in Fig. 3.

When the upper end of secondary I 5 next becomes positive and the lower end negative, tube I8 will iire but the negative potential on the grid cannot extinguish tube 23 since the grid lost control because of the space charge of the free positive ions in that tube. However, as soon as gas tube I8 strikes, the negative end of commutating condenser 33, and therefore its positive end also, is raised by the amount of the voltage drop in resistance 25, and this voltage drop is greater than the internal plate-cathode drop of gas tube 23. Since condenser 33 cannot discharge quickly because of the value of its time constant, cathode 25 assumes a voltage higher than anode 21 and tube 23 is extinguished.

The cathode is held above the voltage of plate 21 for such length of time that the positive ions diffuse from around the grid but the negative potential of the grid prevents the tube from restriking on this Vnegative half cycle. Condenser 34 now starts to charge and condenser 31 to discharge. The charging current extends from 28 to the anode 26, cathode 2|', condenser 34, resistance 35 and ground 33 and thence to the negative terminal 29. Condenser 31 discharges through resistances 24, I3 and 38. These two condensers and their circuits have such time constants'that they charge or discharge fully within a half cycle of the highest frequency used in the sub-carrier frequency modulation. When condenser 34 charges in the way just described rectifier 39 places a positive pulsating potential across resistance 42 due to charging current from anode 40 to ground terminal 43. 'I'he drop in resistance 38 from discharge'oi condenser 31 places a negative potential on rectifier plate 4I. course can flow through this half of the rectifier for this part of the cycle.

For the next half cycle gas tube 23 will strike and gas tube I8 will be extinguished by the action of commutating condenser 33 which need not be further described. As condenser 31 recharges, rectified current will pass through resistance l2 to ground in the way previously referred to, but the potential produced in resistance 35 from discharge of condenser 34 will not send current through the rectifier.

From the described operation it will be apparent that two pulses will pass through load resistance 152 for each complete cycle of the subcarrier frequency in the input transformer. By proper amplification of the signal before demodulation the Thyratron tubes can be made to operate on a very Weak signal such as in a severe fade. The current pulses passing through resistance 42 will be of constant amplitude. In other words, the potential drop in resistancelfl foreach pulse will be independent of the signal strength. The average current in resistance 42 is, however, dependent directly upon the number of pulses per second. The greater the number of pulses per second the higher will be the average current in this resistance. The pulsating output of tube 46 will be smoothed out by lter I3 and the voltage of the input of recorder i4 will vary in amplitude with the varying frequency of the input of the demodulator. Since the output is independent of varying amplitude in the input that may have been produced by fading conditions or by any other cause no voltage limiters are required anywhere in the receiving system. The demodulator inherently acts as a limiter in its conversion of the frequency variations into amplitude variations. The amplitude modulations produced by fading conditions are thus eliminated. At the No current of I assuma same time the demodulator overcomes all the dii.'- iiculties of the fllter type used in the prior art as previously referred to herein.

Instead of W pass filters in the receiving system I may use band pass filters and various other additions and substitutions of devices well known in the art may be made in either the transmitting or receiving system without departing from the spirit of the invention. I

'Ihe amplifier 46 may be omitted and filter i3 connected directly to resistance 42 and the action will be the same. Also, it is permissible to use only one of the condensers 34 and 31. This would halve the number of rectified pulses in the output of the demodulator but the same results could be obtained as with two condensers.

In disclosing the transmitting and receiving system with which my invention is used, I have described only those that are essential for a clear understanding of the invention.' It will be ap parent that in the commercial installation many other well known devices will be used for improved operation of the system as a whole, but these 'do not affect the operation of the demodulator to which my invention relates.

Having described my invention. what I claim is:

l. in a demodulator for a frequency modulated carrier wave, means for receiving the frequency modulated carrierwave, means for storing a predetermined quantity of electrical energy during each half -cycle of the carrier wave and discharging it during the succeeding half cycle, means for rectifying a constant fractional part of the energy thus passing through said first mentioned means and an indicator operated by the rectified energy.

d. In a demodulator for a frequency modulated carrier wave, means for receiving the frequency modulated carrier wave, means for storing a pre` determined quantity of electrical energy during each half cycle of the carrier wave and discharging :it during the succeeding half cycle, means for rectifying a constant fractional part of the energy passing in one direction through the first mentioned means and an indicator operated by the rectied energy. 'l

d. In a demodulator for a frequency modulated carrier wave, means for receiving the frequency modulated carrier wave, a pair .of condensers, means for charging one condenser to a predeter' mined constantpotential on the positive half cycles of the carrier wave and discharging it on the negative half cycles, means for charging the other condenser to a predetermined constant potential on the negative half cycles and discharging it on the positivel half cycles, means for rectifying a constant fractional part of the current passing through said condensers and an indicator operated by the rectified energy.

t. In a demodulator for a frequency modulated carrier wave, means for receiving the frequency modulated carrier wave, a pair of condensers, means for charging one condenser to a predetermined constant potential on the positive half cycles of the carrier wave and discharging it on the negative half cycles, means for charging the other condenser to a predetermined constant potential on the negative half cycles and discharging it on the positive half cycles. means for rectifying a constant fractional part of the current passing through said condensers, a nlter lll for smoothing the rectified current, and an in- .dicator operated by the rectified current.

5. In a demodulatorfor a frequency modulated carrier wave, means for receiving the frequency modulated carrier wave, a pair of condensers, a pair of resistances, means for charging one condenser to a predetermined constant potential on the positive half cycles of the carrier wave and discharging it through one' resistance on the negative half cycles, means for charging the other condenser to a predetermined constant potential on the negative half cycles and discharging it through the other resistance on the positive half cycles, a rectifier connected around each resistance and an indicator operated by the current passing through the-rectifiers.

6. In a demodulator for a frequency modulated carrier wave, means for receiving the frequency modulated carrier wave, a pair of condensers, a pair of resistanceameans for charging one condenser to a predetermined constant potential on the positive half cycles of the carrier wave and discharging it through one resistance on the negative half cycles, means for charging the other condenser to a predetermined constant potential on the negative half cycles and discharging it through the other resistance on the positive half cycles, a rectifier connected around each resistance, a filter connected to the rectiflers and an indicator operated by the current passing through the filter.

7. In a demodulator-for a frequency modulated carrier Wave, means for receiving the frequency modulated carrier wave, a pair of gas tubes each having a cathode, a control electrode, and an anode, a signal input source connected to the control electrode and cathode of the tubes in phase opposition to fire each tube alternately and on alternate half cycles, means to extinguish one gas tube on one-half cycle when the other hres on the other half cycle, a condenser, means connected to said tubes for charging the last mentioned condenser to a predetermined constant value on one-half wave of the carrier Wave and to discharge it on Fthe succeeding half wave, means for rectifying a constant fractional part of the alternating current passing through the last mentioned condenser and indicating means responsive to the average value of said rectified current.

8. In a demodulator for a frequency modulated carrier wave, means for receiving the frequency modulated carrier wave, a pair of gas tubes each having a cathode, a control electrode and an anode, a signal linput source connected to the control electrode and cathode of the tubes in phase opposition to fire each tube alternately and. on alternate half cycles, means to extinguish one gas tube on one-half cycle when the other lires on the other half cycle, a condenser, means connected to said tubes for charging the last mentioned condenser to a predetermined constant value on one-half wave of the carrier wave and to discharge it on the succeeding half wave, means for rectifying a constant fractional part of the alternating current passing through the last mentioned condenser, means to integrate the rectified pulses and indicating means responsive to the output of the integrating means.

GILBERT R. CLARK. 

